In semiconductor device fabrication, the thickness of the formed film needs to be reduced as the size of the semiconductor device becomes miniaturized. However, it may be difficult to form a thin film in cases where an impurity is doped into silicon. For example, it is difficult to form a thin silicon film doped with boron, because it is difficult to control the film thickness uniformity during the initial stage of the process. Also, the silicon atoms existing on the surface of the film flow significantly as a result of boron doping, causing aggregation and making it difficult to form a film with uniform thickness.